1. Field of the Invention
The present invention relates to an ink jet printing apparatus which forms an image by applying to a print medium an ink containing a colorant that coagulates under a predetermined condition. More specifically, the present invention relates to a construction that prevents problems caused by a coagulation of the colorant on a print medium.
2. Description of the Related Art
As office equipment such as computers, word processors and copying machines advance, a growing number of printing apparatus for outputting information from these equipment has become available on the market. The printing apparatus employing an ink jet printing system in particular has an advantage of being able to reduce the size of a print head easily, print an image at high resolution and high speed and print on plain paper without requiring special processing on the paper. Other advantages include low running cost, low noise and a relative ease with which a full color printing can be realized using multiple color inks. It has therefore found a wide range of applications, including personal users.
Such a widespread use can lead to the user making new demands on the ink jet printing apparatus. In recent years in particular, there are growing calls for increased image fastness such as waterfastness and lightfastness while maintaining a high color saturation. One method of enhancing the image fastness is to make some improvements on the print medium as dedicated paper. However, to stably maintain a high image fastness of various kinds of print mediums including plain paper, it is more effective to provide an ink itself with some features to achieve the above objective. For this reason, recent years have seen many novel inks developed and their applications proposed.
For example, Japanese Patent Application Laid-open No. 11-227229 (1999) discloses, in addition to the conventionally used dye inks, the development of inks containing pigments as colorants and a variety of printing methods using such inks. The inks containing pigments tend to stay on the surface of a print medium with the colorants in a coagulated state, when compared with inks containing dyes as colorants. Thus, the pigment colorants have features of a high color saturation which is not easily faded by sunlight and ozone. To take advantage of both the superiority of the pigment ink and the superiority of the dye ink, the above-cited reference discloses a method that selectively uses these different kinds of inks according to the kind of print medium used and the kind of image to be output. For example, the above document describes that a pigment-based black ink with low penetrability and dye-based color inks with high penetrability are prepared and that a black image may be printed with the black ink or with a combination of different color inks, depending on the kind of print medium and the kind of image to be printed. The cited reference also describes printing color inks first, followed by a black ink overlapping the first printed color inks.
Other methods for enhancing the color saturation and the image fastness propose using a reaction liquid that reacts with color inks containing colorants to make the colorants insoluble or coagulate. For example, Japanese Patent Application Laid-open No. 56-89595 (1981) discloses a method which applies a polymer solution, such as carboxymethyl cellulose, polyvinyl alcohol and polyvinyl acetate, to the print medium before printing and then prints coloring inks. Japanese Patent Application Laid-open No. 63-29971 (1988) discloses a method that involves applying to a print medium a liquid containing an organic compound having two or more cationic groups in one molecule and then printing coloring inks containing anionic dye. Japanese Patent Application Laid-open No. 64-009279 (1989) discloses a method that first applies an acidic liquid containing succinic acid to a print medium and then prints coloring inks. Japanese Patent Application Laid-open No. 64-063185 (1989) describes a method that applies to a print medium a liquid that makes a dye insoluble, before printing coloring inks containing the dye. Japanese Patent Application Laid-open No. 5-202328 (1993) describes a method which applies a reaction liquid containing polyvalent metal ion before printing coloring inks.
Further, Japanese Patent Application Laid-Open Nos. 6-106841 (1994), 9-118850 (1997), 11-334101(1999), and 11-343441(1999), and U.S. Pat. Nos. 5,428,383, 5,488,402 and 5,976,230 disclose a set of a black ink and color inks in which at least one of the color inks exhibits a mutual reactivity with the black ink, with other inks showing no reactivity with the black ink.
All these methods listed above that use a reaction liquid are characterized in that the reaction liquid chemically reacts with the coloring inks containing colorants to coagulate the coloring inks. That is, many ink jet printing apparatus of recent years, whether they use pigments or dyes or whether they require a reaction liquid to induce coagulation, have the colorants coagulate, remain and settle on the surface of a print medium, thereby realizing a satisfactory color saturation and image fastness.
However, the coagulation of colorants described above is not always desirable. For example, when an ink containing a colorant with a coagulating property is applied to glossy paper, a new problem arises. The ink with a coagulating nature tends to stay on the surface of the print medium and form relatively large undulations. Such large undulations on the print medium surface degrade the glossiness of the print medium. Not only can the printed paper in such a state not take advantage of the characteristic of the glossy paper, but a difference in glossiness between the printed portions and the non-printed portions becomes conspicuous, giving the user an impression of unnaturalness.